Compliant hard fusing pressure roller for reducing media curl and wave in electrophotographic printers

Abstract: A compliant hard fusing pressure roller (51) for reducing media (12) curl and wave in electrophotographic printers (10) as part of an improved fuser apparatus. The improved fuser consists of the standard components of a fuser, namely of a heated fuser roller (26) and one or more pressure rollers (51). The pressure roller (51) is constructed with a central rotation shaft (53) with a cylindrical deformable material (52) such as silicone rubber is formed around the central rotation shaft. However, a cylindrical hard outer shell (54) is formed around the deformable material (52). The compliant hard pressure roller exerts a tangential force toward the fuser roller (26). Because the outer hard shell (54) of the compliant hard pressure roller (51) is formed from a malleable type material, it provides some compliance and therefor deforms slightly from roundness for small, sharp errors in the fuser roller (26) or media (12). Larger errors such as out of roundness, or miss alignment between the compliant hard pressure roller (51) and the fuser roller (26), are absorbed by the cylindrical deformable material (52). Both the pressure (51) and heated fuser rollers (26) are rotated about their respective axis. The printed media (12) is compressed between the pressure roller (51) and the heated fuser roller (26) thereby fusing the image onto the printed media (12) while the media (12) is transported within the printer (10).

Description: Technical Field This invention relates generally to electrophotographic printing also known as laser printing and more particularly to an improved fusing roller for use in the output paper path of the desk top type laser printer. This arrangement is useful to reduce curl and wave produced by the fuser system used in these printers. Background of the Invention With present technology laser printing systems, it is common practice to pass the printed media or paper with the just printed text or graphics between a fuser roller and idler pressure roller to burn in or fuse in the text or graphics on the media. This eliminates the possibility of smearing the media thus enhancing the permanent nature of the generated document. As the media leaves the surface of the photoconductive drum and transfer rollers, it is guided into the fuser. In the fuser the media is compressed between a pressure roller and a fuser roller. The fuser roller is heated to a temperature of about 180 degrees Celsius. At this temperature, the toner liquefies thereby fusing to the media. To insure high print quality, the toner must be evenly fused to the paper. To allow for machining tolerances and assembly errors in the fuser roller itself, the pressure roller has generally been, prior to the present invention, made of a compliant material. Because the pressure roller is made of such material it tends to deform as it presses against the fuser roller. As the paper passes between the fuser roller and a pressure roller, a fixed amount of the media is held in contact with the fuser roller. This point of contact is also known in the art as nip. While the compliant pressure roller may reduce manufacturing tolerances in the fuser roller itself, it also tends to widen the nip width on the printed media. If a completely non-compliant pressure roller is used, the nip width can be reduced to a minimum. However, with this arrangement the necessary manufacturing tolerance on the fuser assembly makes the system unmanufacturable in mass quantities. If the tolerances are not accounted for, the pressure seen by the media may vary significantly both longitudinally and laterally along the media thus, the print quality is adversely affected. As the nip width increases, the amount of the print media in contact with the hot fuser roller increases. This translates into a proportional amount of the print media being subjected to the curvature of the fuser roller. Thus the print media is simultaneously exposed to high temperature and curve. As the print media and toner cool, the media is susceptible to retaining the contour of the fuser roller. The net result of this operation is the production of an undesirable curl in the print media. This curl may then hinder subsequent paper handling devices such as photocopying, facsimile machines or the like. If the nip width is not maintained across the media and down the media, the media tends to deform in a different manner. This uneven nip width, which is generally caused by a varying pressure between the pressure roller and the fuser roller, introduces a characteristic wave in the print media. Wave differs from curl in that wave generally is more of an oscillating type distortion. However, wave can also hinder subsequent paper handling devices. Additionally, a wave gives a document stack a less than professional appearance. JP-A-59 172 668 discloses a pressure roller for fixation. The pressure roller comprises a foamed body layer of silicone rubber, the outer circumference of which is covered with a surface layer of fluororesin which deforms freely in a thin cylinder shape. If the pressure roller contacts a heat roller on a specific nip pressure, it deforms easily. The surface layer of fluororesin is thin, so that the roller deforms freely making the nip width large. The surface layer of fluororesin has a hardness relatively higher than that of the foamed body. It is the object of the present invention to provide a pressure roller that is compliant yet exhibits a significantly smaller nip width of the printed media while maintaining the same pressure and thereby reducing both curl and wave introduced into the print media. This object is achieved by an apparatus according to claim 1. By using a harder pressure roller, the nip width can be reduced while maintaining the same pressure against the fuser roller. As an additional benefit, the hard pressure roller allows for tighter control of the nip width along the print media. Thus, by reducing and stricter control of the nip width, the characteristic curl and wave normally induced into the print media are greatly reduced. Another benefit provided by the present invention is the ability to easily create specific pressure profiles exerted by the pressure roller. Various profiles can be obtained by simply removing portions of the deformable material under the hard outer shell. For example, by removing deformable material from the ends, the pressure across the pressure roller can be equalized. Brief Description of the Drawings FIG. 1 is a cut away isometric view of an electrophotographic printer housing showing the paper path through the fuser in accordance with the present invention. FIG. 2 is a cross sectional view of the fuser roller and pressure roller with the widened nip width. FIG. 3 is a cross sectional view of the fuser roller and pressure roller showing the reduced nip width in accordance with the present invention. Detailed Description Of The Preferred Embodiments Referring first to Fig. 1, where a desk top laser printer 10 with a cutaway view is shown. For simplification of FIG. 1, much of the mechanical detail has been omitted. One skilled in the art of electrophotographic printing will understand that this figure is a simplified diagram used to orient the reader as to the function of the present invention. To print an image, print media 12 is extracted from a paper supply not shown in FIG. 1. The image is first imaged onto photoconductive drum 20 by techniques understood by those skilled in the art. As the media passes between photoconductive drum and transfer roller 22, the image is transferred from the photoconductive drum to the media 12. Media 12 next is guided to fuser 24 after it leaves the photoconductive drum 20 and transfer roller 22. Figure 1 shows that the media is sandwiched between the fuser roller and the pressure roller as it passes through the fuser. Because of the high temperature of the fuser, the image is fused onto the print media. Print media 12 leaves the fuser and continues along the paper path guided by suitable media guide means such as guide 50. Other guide means are not shown in FIG. 1. Print media 12 is finally deposited in the output tray 14. One embodiment of the present invention uses a heated fuser roller 26 and single pressure roller 28. Other embodiments may use more than one pressure roller. Referring next to Fig. 2, there is shown enlarged cross sectional view of the fuser roller 26 and idler pressure roller 28 as described above. The fuser roller 26 includes a quartz lamp 37 mounted at the center axis. Radiant heat from the quartz lamp 37 travels through open space 38 to heat the outer cylindrical roller member 40. The pressure roller 28 has a central rotation shaft 41. Shaft 41 is coated with a deformable material 42 thereby giving pressure roller 28 compliance. While many materials may be used as the deformable material, it must be remembered that the hot roller 40 is maintained at a relatively high temperature. One particular material that has been found to withstand the pressure and temperature is silicone rubber. Generally, the deformable material will have a finished layer 44 place over it to reduce the probability that the media will adhere to the pressure roller. As can be seen in Fig. 2 the deformable silicone rubber 42 flattens out or deforms under the pressure exerted against the hot roller 40. As a result of this pressure, media 12 will be subjected to a slight curvature of the hot roller 40 as it passes through the nip 48. The actual nip width is an engineering decision and varies from fuser to fuser; a common fuser might use a nip width of approximately 3mm. Such an assembly is for example disclosed in US-A- 5,245,393. Referring next to Fig. 3, there is shown an enlarged cross sectional view of the fuser roller 26 and compliant hard idler pressure roller 51 of the present invention. As with the prior art, the present invention pressure roller includes a central rotation shaft 53 and again is coated with deformable material 52 such as silicone type rubber. Over the top of the silicone rubber 52 is placed a hard shell 54. This hard shell can be made of many materials. In the present invention brass was chosen because it is fairly rigid with some compliance and not brittle. Other materials also exhibit these properties, but may not be as easily machined to fit over the silicone rubber 52. If, however, the outer hard shell 54 is not intended to slip over the silicone rubber 52 but instead intended to have the silicone rubber injected between the outer shell and the central rotation shaft, then machining may not be a significant concern. Still referring to Fig. 3, pressure roller 51 exerts a tangential force toward the fuser roller 40. Because the outer hard shell 54 of the pressure roller is formed from a malleable type material, it provides some compliance and therefor deforms slightly from roundness. However, the hard outer shell significantly retains it roundness compared to the pressure roller of FIG. 2. The hard outer shell will deform for small, sharp errors in the fuser roller 40 or media 12. Larger errors such as out of roundness, or miss alignment between the pressure roller 51 and the fuser roller 40, are absorbed by the silicone rubber. Thus, the new pressure roller actually does exhibit some compliance however, the compliance motion is dispersed between the hard outer shell and the softer interior silicone rubber. As a result of pressure roller 51 significantly retaining its roundness, the nip width and thus the amount of media in contact with hot roller is greatly reduced. While using a slip-on brass tube or sleeve with a thickness of about 0.3 mm, the nip width has been reduced by approximately one half while maintaining the same fuser pressure of the compliant pressure roller. Additionally, with the hard outer shell, the pressure exerted by the pressure roller along the length of the pressure roller is applied in a more even manner. This better control of the pressure applied to the media along with the reduced nip width reduces the amount of wave introduced into the media. Not shown in Fig. 3, it has been determined that placing a coating of a non-stick surface around the hard shell of the pressure roller greatly reduces the chances of the print media adhering to the pressure roller. The hot roller 40 may also have such a coating placed on it. By using the hard outer shell, the pressure applied against the fuser roller can be contoured to best fit the particular application. In the typical fuser, the pressure roller is end mounted to the fuser assembly. Such a mounting creates a higher pressure on the ends of the roller. By removing some of the silicone rubber under the hard shell, the pressure can be equalized across the fuser. Other applications may require a particular pressure profile. Such profiles can easily be obtained by simply removing sections of the silicone rubber. Although the preferred embodiment of the invention has been illustrated, and that form described, it is readily apparent to those skilled in the art that various modifications may be made therein without departing from the scope of the appended claims.

Claims: An apparatus operative within an electrophotographic printer (10) for reducing deformation in electrophotographically printed media (12), said apparatus comprising : a heated fuser roller means (26) on a first side of a media transport path, said heated fuser roller means (26) being rotated about an axis of rotation longitudinally along an axis of said heated fuser roller means; a pressure roller means (51) on a second side of said media transport path and positioned adjacent to said heated fuser roller means (26), said pressure roller means (51) comprised of a central rotation shaft (53), a cylindrical deformable material (52) formed around said central rotation shaft (53), a cylindrical hard outer shell means (54) formed around said deformable material (52), said pressure roller means (51) being rotated about an axis of rotation longitudinally along an axis of central rotation shaft (53), said printed media (12) being compressed between said pressure roller (51) and said heated fuser roller (26) thereby transporting said printed media (12) within said printer characterized in that said cylindrical hard outer shell means (54) is constructed from a hard malleable material, so as to significantly retain its roundness under pressure. The apparatus as claimed in claim 1 wherein said cylindrical deformable material (52) is silicone rubber. The apparatus as claimed in claim 1 wherein said malleable material is brass. The apparatus as claimed in claim 1 wherein said cylindrical deformable material (52) is silicone rubber, said cylindrical hard outer shell means (54) is constructed from a brass formed around said silicone rubber material, and said printed media (12) has a first side on which an image has been applied and a second side, said first side being in contact with said heated fuser roller means (26) and said second side being in physical contact with said brass hard outer shell (54) of said pressure roller means (51), heat from said heated fuser roller means (26) and pressure from said pressure roller means (51) fuses said image to said printed media. The apparatus as claimed in claim 4 further including a media guide means (50) arranged to receive said printed media (12) and deflect said printed media's (12) travel to allow further processing of said printed media (12) by the printer (10). The apparatus as claimed in any of the preceding claims, wherein said cylindrical deformable material (52) is removed from specific portions of said pressure roller means (51) such that a particular pressure profile is obtained.

Inventors: HUFFMAN JOHN W, MUI PAUL K, HUFFMAN, JOHN W., MUI, PAUL K.
Applicants: HEWLETT PACKARD CO, HEWLETT-PACKARD COMPANY
Application year: 1994
Main classification: G03G15
IPC codes: B65H23, G03G15